FIG. 1 shows a prior art overrunning alternator decoupler 10. The main components of the prior art decoupler 10 are:                a pulley 12;        a bearing 14;        a bearing cover 16;        a bushing 17;        a torsion spring 18;        a driving clutch member (in this case a drum) 20;        a one-way wrap spring clutch 24;        a driven clutch member (in this case a drum) 28; and        a hub 30.        
In the prior art decoupler 10 the torsion spring 18 is connected in series with the one-way wrap spring clutch 24. In operation, torque is supplied from the engine accessory drive belt (not shown) to rotate the pulley 12 in a first direction. The inner diameter of the pulley 26 has a feature (not shown) that abuts an end of the torsion spring 18 to transfer force thereto. The other end of the torsion spring 18 is connected to the driving clutch drum 20 (which is not fixed to the pulley 12 but mounted to rotate about an axis defined by the hub 30), thus transferring torque from the pulley 12 to the driving clutch drum 20 via the torsion spring 18. The driven clutch drum 28 is concentrically mounted with the driving clutch drum 20 and the one-way wrap spring clutch 24 is mounted within driving clutch drum 20 and the driven clutch drum 28, i.e., the volutes or coils of the wrap spring clutch 24 extend along the inners bores of the drive and driven drums 20, 28. Thus when the torque is transferred in the first direction (which is capable of being reacted by the one-way wrap spring clutch), the wrap spring clutch coils transfer torque from the driving clutch drum 20 to the driven clutch drum 28. The driven clutch drum 28 is fixed to the hub 30, thus transferring power to the alternator.
When the engine decelerates rapidly, for example during a wide-open throttle shift, the inertia of the alternator causes the hub to over speed the pulley causing the torque to reverse direction. In this case the one-way wrap spring clutch does not couple the torque from the hub and driven clutch drum to the driving clutch drum, whereby the hub and driven clutch drum freewheel relative to the driving clutch drum, torsion spring, and pulley, and vice versa.